The present invention relates in general to transmission-lines, and more particularly to microstrip transmission-line cables with two or more microstrip signal lines.
Microstrip transmission-lines are commonly used for high-bandwidth (&gt;1 Ghz) or high data-rate (&gt;300 Mb/s) signal input/output subsystems. When multiple microstrips are combined in a cable, microstrip-to-microstrip interference (crosstalk) between adjacent channels often becomes a dominant cause of signal degradation and noise. The structure of a typical microstrip transmission-line includes two or more microstrip signal lines, a ground plane, a separating base dielectric, and an optional coverlay.
Far-end crosstalk refers to the crosstalk mechanism resulting from disparate propagation velocities of the various propagation modes of the microstrip transmission-line. When the signal asserted on a driving channel of the microstrip transmission-line excites multiple propagation modes, they recombine imperfectly at the far end of the cable resulting in difference signals (crosstalk) on the neighboring lines. Far end crosstalk is dominant in microstrips with lengths on the order of the wavelength of the highest frequency signal of interest or longer.
Addition of a coverlay to a microstrip transmission line reduces far-end crosstalk. Currently, to increase the reduction in crosstalk of microstrip transmission lines, the physical thickness of the coverlay material is increased. This solution is limited since the coverlay thickness must be increased infinitely to completely eliminate crosstalk, and this causes the cable to increase in thickness and decrease in flexibility.
Another type of multi-signal transmission line, disclosed in U.S. Pat. No. 3,763,306 U.S. Pat. No. Re. 31,477 (Marshall), uses a jacket made of a dielectric material having a higher dielectric constant than the dielectric core material of the cable, thus reducing far-end crosstalk, but not eliminating it. Also, microstrip transmission-line cables are structured differently and optimally have a coverlay only on one side for high flexibility and low cost of manufacture. Conversely, the cable disclosed in Marshall is manufactured using an extrusion process. Optimally, microstrip transmission-lines are manufactured as flexible printed wiring boards because of the low cost of production and their specific use as extremely compact, ribbon cable interconnects between modules.
What is desired is a microstrip transmission line cable with no far-end crosstalk, with practical, finite coverlay thickness, and manufactured as a flexible printed wiring board.